The engineer and mechanist employ nearly the same routine for polishing these three materials, more particularly in turned works, in which the variations principally depend upon the degree of finish required. This general article is therefore intended to apply to each of the three materials; and some particular observations expressly suited to each of them, will be found under their respective heads of Wrought-Iron, Cast-Iron, and Steel.

Turned Works

1. - Large Sized Turned Works. - Such parts of machinery as come under this denomination, are in almost every case turned in self-acting lathes, which, under proper management, leave the surfaces very exact and smooth, so that many of them require no polishing whatever; and which process is reserved for those exterior parts which meet the eye, when the machinery is erected.

Heavy works are made to revolve with considerably greater velocity than that proper for turning, and they are polished with a long stick of deal 1 to 2 inches thick, and 2 to 4 inches wide, the end of which is cut off square. The stick is dipped into a shallow vessel containing oil, then into another with dry emery after which it is pressed forcibly against the work, never being allowed to remain long in one position upon the lathe rest. Occasionally, for additional purchase, a bent bar of iron is used, to the end of which is fixed a block of wood, in imitation of the hanging tools for turning iron, figs. 423 and 424, page 527, vol. ii. Sometimes on the end of the polishing stick is placed a thick piece of leather for the application of the emery, of which two or at most three different-sized grains are used, namely, corn emery, grinding, and fine grinding emery. 2. - Medium sized Turned Works. - Many of these which are turned in power lathes running at a proportionate velocity, with tools properly formed and lubricated with abundance of water from a small jet, are left so smooth as hardly to want any polishing, or at most an inconsiderable amount of polishing with fine emery powder or emery paper; but in other works less skilfully turned by hand tools and with little or no water, it is usual to reduce any very trifling irregularities of surface to a general level, by means of a smooth file, slightly greased, which is rubbed lightly over the work as it revolves; careless workmen are apt however to rely too much on this practice, and having left the work full of ridges from the turning tool, to begin with a coarse file; this practice is detrimental to the production of good true work, and the preservation of the angles.

Works of medium size are polished nearly as above described, but with a deal stick chopped to a chisel edge, or to a square point and thrust against the work; sometimes instead of the point the side of the stick near the end is used as a crow-bar for additional purchase.

Generally two, but occasionally three sizes of emery are used, varying from grinding to flour emery; but it is necessary between the application of each powder, to wipe the work entirely clean, with rags, cotton-waste, sawdust, moslings, (or the curriers' shavings of leather), and also to use a fresh stick, or to chop a clean point, for every kind of emery.

3. - Small sized Turned Works. - For these, emery sticks, (those with emery glued upon them,) and emery paper are much used; but the loose powder applied as above although less cleanly, is in general somewhat quicker and also cheaper. For the plane surfaces and other parts of small-turned works, required to be particularly flat, emery paper folded around a smooth file or a flat piece of wood is used, or else flat pieces of mahogany, box-wood or metal, supplied with fine emery powder and oil are employed with still greater advantage.

In some few cases after the finest or flour emery has been used, fine crocus is applied similarly, or with a buff stick, but this is unusual as two sizes of emery are alone in general employed. Some parts of superior works in iron and steel, especially the rounded edges, are brightened with the burnisher, but such parts require to be previously polished quite smooth; both the work and burnisher must be wiped thoroughly clean from emery or dust, the burnisher is then held against the work as it revolves, a little oil being interposed to lubricate the surfaces.

4. - Screw Threads that are required to fit accurately and smoothly, and also to sustain frequent unscrewing, should be polished with a pointed stick and emery; as frequently the removal of the rough edges will make that screw enter which appeared to be too large, and the smooth screw present far less friction and disposition to wear out.

5. - The Heads of Screws are often finished with the side of an emery stick as they revolve in the lathe; and if they are to be burnished the emery must be carefully removed from the notch by folding the rag and drawing it through like a saw or the process will fail, and the burnisher will be injured.

6. - Small Round Rods used for inferior purposes and not requiring to be cylindrical, are often ground bright against the edges of large revolving grindstones driven by power. The rod is held rather loosely in the hands of the workman and at a small angle to the axis of the stone; then without any great attention on the part of the individual, the grindstone causes the rod slowly to rotate in his hands, so as to act on every part of its circumference, and the obliquity of the two axes also causes the rod to traverse endlong through the hands like a screw, and thus every part of the rod is acted upon successively by the grindstone.

7. - Cylindrical Works that require great accuracy are ground by methods that will be explained in Chap. XXXIII. Section 2, but other cylindrical rods of inferior kinds, used only as levers and for similar common purposes, are often polished between two sticks, (supplied with emery and oil,) placed transversely to the cylinder, grasped in both hands, and rubbed lengthways on the work as the lathe revolves. Considerable friction may thus be given on opposite sides, and therefore without bending the cylinder, the figure of which is materially improved by the treatment. Sometimes for greater purchase, the sticks are united at the one end by a loop of string or wire, and compressed at the other, like nut-crackers, with one or both hands. 8. - Lathes fob Polishing. - In large manufactories it is usual to perform the polishing on common lathes kept entirely apart from those used for turning, on account of the mischief that ensues when gritty matters find their way into the fittings of the mandrel or other part of the lathe; and careful workmen who use the same lathe for turning and also for polishing, avoid with scrupulous care the scattering of the powders, and frequently employ a spare center for the popit head, in polishing spindles and pieces requiring support at both ends, as the grit is almost sure to deteriorate the center employed in polishing works.